NEW YORK — A COVID-19 vaccination requirement for teachers and other staff members took effect in New York City's sprawling public school system Monday in a key test of the employee vaccination mandates now being rolled out across the country.

Mayor Bill de Blasio said 95% of the city's roughly 148,000 public school staffers had received at least one vaccine dose as of Monday morning, including 96% of teachers and 99% of principals.

Some 43,000 employees have gotten the shots since the mandate was announced Aug. 23, de Blasio said. “Our parents need to know their kids will be safe," the mayor said. "They entrust us with their children. That’s what this mandate is all about. Every adult in our schools is now vaccinated, and that’s going to be the rule going forward.” U.S. Secretary of Education Miguel Cardona joined de Blasio's virtual briefing and hailed the vaccine mandate. “You’re doing it right," Cardona said. “Students need to be in the classroom. They need to be safe and we need to make sure we’re doing everything possible to let our staff get vaccinated and make sure that our schools are as safe as possible.” The mayor had warned that unvaccinated school employees would be placed on unpaid leave and not be allowed to work this week. The city planned to bring in substitutes where needed.

Schools Chancellor Meisha Ross Porter said she did not know exactly how many employees had declined the shots and been put on leave.

Implementing the mandate smoothly will be a test for de Blasio, a Democrat who boasted of the city’s record of keeping school buildings open during most of the last school year when other districts went to all-remote instruction. New York City is not offering a remote option this year.

The vaccination mandate in the nation’s largest school system does not include a test-out option, but does allow for medical and religious exemptions. It was supposed to go into effect last week but was delayed when a federal appeals court granted a temporary injunction. An appeals panel reversed that decision three days later.

The 96% teacher vaccination rate cited by the mayor was slightly different from the 97% figure provided earlier Monday by United Federation of Teachers head Michael Mulgrew.

Vaccination rates rose in every school job category after the mandate was announced. District Council 37 of the American Federation of State, County and Municipal Employees, which represents some 20,000 city Department of Education employees, said 93% of those workers had provided proof of at least one COVID-19 vaccine dose as of Sunday, up from 68% at the beginning of September. Under a deal worked out with the city, unvaccinated District Council 37 members who want to apply for a medical or religious exemption have until 5 p.m. Tuesday to do so, union officials said in a news release. New York City's million-plus-student public school system is one of the first in the nation to require inoculations for all staff members. A similar mandate is set to go into effect in Los Angeles on Oct. 15.

A group of teachers and other school employees who had sued over New York's school vaccine mandate asked the U.S. Supreme Court on Thursday for an emergency injunction blocking its implementation. The request was denied Friday.

Many students and parents support the vaccine mandate as the best way to keep schools open during the pandemic.

“It’s safer for our kids,” said Joyce Ramirez, 28, who was picking her three children up from a Bronx elementary school last week.

Ramirez said she hopes the requirement will lessen the chances of teachers contracting the virus and prompting classroom or school shutdowns.

Cody Miller, a 15-year-old sophomore at a high school in Manhattan, said teachers should all be vaccinated. “I think they should,” said the teen, who got vaccinated himself as soon as the Pfizer shot was approved for people 12 and up. “It’s so many kids, it’s a big environment, you know?” But Mally Diroche, another Bronx parent, had mixed feelings. “I kind of feel like that’s a decision they should be able to make on their own,” said the mom of three boys between 3 and 12. Diroche, 29, said she feels that masks and other precautions can check the spread of the virus within schools.

Some educators have reservations about the mandate but are complying.

Maurice Jones, 46, a support staff member at a Manhattan middle school, said he got vaccinated months ago but sympathizes with co-workers who have not gotten the shots. “If they’ve got to get tested more they’ve got to get tested more,” Jones said. “I don’t think they should lose their job.” Roxanne Rizzi, who teaches technology at an elementary school in Queens, waited until Friday to get her first coronavirus vaccine shot.

“I had to do it for the finances of my family,” she said.

Rizzi, 55, had resisted the vaccine because she contracted COVID-19 in November and believed natural immunity would protect her. She said she would continue to protest the mandate.

According to the U.S. Centers for Disease Control and Prevention, people should get vaccinated even if they have already been infected by the virus. The agency says COVID-19 vaccines offer better protection than natural immunity and help prevent getting infected again.

Associated Press writer Jennifer Peltz contributed to this report.